The present invention relates to a new and improved hydraulic control valve for a hydraulic power steering system and more specifically to a hydraulic control valve having a torsion-tension spring that resists relative rotation between a pair of valve members.
In a known power steering system, a torsion bar is utilized to interconnect a pair of relatively rotatable hydraulic valve members. When a steering maneuver is made, the torsion bar is subjected to torsional, elastic deformation. As the torsion bar is elastically deformed, the valve members are rotated relative to each other. Relative rotation between the valve members allows pressurized hydraulic fluid flow from a pump to a hydraulic power steering motor connected between the frame of the vehicle and the steerable wheels to effect operation of the power steering motor and steering of the vehicle. Power steering systems in which a torsion bar is subjected to torsional deformation, are disclosed in U.S. Pat. Nos. 3,709,099; 4,557,342 and 4,598,787.
During operation of a vehicle, it is desirable to have a force which biases the hydraulic valve of a steering system to a neutral, non-steering condition when operator input steering torque is low or nonexistent. It is also desirable to have the amount of manual input torque required to turn the steerable vehicle wheels to be comparable whether a parking maneuver is made when the vehicle is stationary or operating at very low speeds or in a lane change maneuver at 55 miles per hour. However, the amount of torque which a driver of a vehicle must input to the steering system is approximately 240% greater during steering in a parking maneuver than during a lane change maneuver when a vehicle is moving at 55 miles per hour. Torsion bar springs have essentially linear spring rates, and provide a torque biasing the hydraulic control valve toward an on center condition which is the product of angular displacement multiplied times the spring rate. It is desirable to provide a preload to prevent angular displacement of the torsion bar until the input torque reaches a predetermined minimum level.